1. Field of the Invention
This invention relates to automatic exposure control systems for photographic cameras, and more particularly to digital exposure control systems for controlling the diaphragm aperture of a camera in accordance with a preselected shutter time.
2. Description of the Prior Art
In a known automatic exposure control system the setting of a desired shutter time is followed by derivation of a proper exposure value to which the diaphragm aperture of a camera is adjusted by a control mechanism. Though the exposure value is obtained to a three-figure accuracy, the use of the control mechanism causes the actual diaphragm value to deviate from the exposure value by about a half stop. For example, let it be assumed that the object brightness level Bv, the preselected shutter time Tv, the sensitivity of used film Sv and the full open aperture Avo are:
Bv=4.25, Tv=7, Sv=5, and Avo=1.
Then, based on the following formula in the Apex system, the number of stops Avs the diaphragm is to be closed down from the full open aperture is:
______________________________________ Avs = Bv - Tv + Sv - Avo = 4.25 - 7 + 5 - 1 = 1.25 ______________________________________
Therefore, the diaphragm of the camera must be closed down 1.25 stops from the fully open position. If the diaphragm control mechanism operates with an accuracy of 0.5 stops, the actual size of diaphragm aperture is taken at either one stop or one and a half stops down from the fully open position. Hence the risk of exposure error is 0.25 stops.
In another conventional exposure control system, when the computed exposure value exceeds the limits of the dynamic range of diaphragm control, the excess fraction of the exposure value is fed back to alter the preset value of shutter time to produce a correct exposure. However, such adjustment of the shutter time occurs only when the exposure value falls outside the dynamic range of diaphragm control. Accordingly, the above-described risk of exposure error is not reduced to any significant extent.